1. Field of the Invention
This invention relates to oversampling techniques in general and, more particularly, to oversampling sigma-delta modulators.
2. Description of the Prior Art
Sigma-delta (.SIGMA.-.DELTA.) techniques (as part of the digital-to-analog or analog-to-digital conversion function) are finding wide acceptance in many applications, such as telephone codecs, compact disc (CD) players and the like. .SIGMA.-.DELTA. techniques are popular because of the tolerance of the techniques to circuit variations present in integrated circuit fabrication processes. Hence, a sixteen or more bit linear converter may be implemented relatively inexpensively in integrated form, compared to more conventional circuit techniques such as flash converters or subranging converters.
.SIGMA.-.DELTA. converters are not without drawbacks, however. High bit rate processing is required, pushing low power technologies (such as CMOS) to their limits, especially with wide bandwidth signals such as digital audio. In addition, .SIGMA.-.DELTA. converters suffer from periodic noise and spurious tone generation (in-band and out-of-band) due to the feedback required to implement the converter, which will be discussed in more detail below. Although the periodic noise and spurious tones typically occur at very low levels (for example, at about 90 dB below full scale), they may be very objectionable to a human listener while having virtually no impact on a data acquisition system using the same converter. The noise and tones are typically noticeable to a human listener when no, or a very low, desired signal is present. The periodic noise and tones are generally referred to as idle channel noise.
Prior art techniques for removing the periodic noise and tones generally attempt to "whiten" the periodic noise and tones from the converter, thereby suppressing them. These techniques include adding a small dither signal (noise) or an out-of-band tone (such as a 25 KHz sine wave, which is above the human ear's hearing frequency range) to the input to the .SIGMA.-.DELTA. converter. Generally, the addition of the dither signal is not regarded as wholly effective since it adds noise to the output of the converter (which may raise the noise floor of the converter) while not entirely suppressing the periodic noise and spurious tones. While the out-of-band tone insertion may reduce the in-band spurious tones, the dynamic range of the converter suffers since the converter now has to process the desired signal and out-of-band tone without saturating.